High efficiency transverse-field traveling wave tube having fast wave dissipative coupler between interaction circuit and collector for decelerating electrons



Jan. 31, 1967 5.1 UDELSON 3,302,053

HIGH EFFICIENCY TRANSVERSE-FIELD TRAVELING WAVE TUBE HAVING FAST WAVE 'DISSIPATIVE COUPLER BETWEEN INTERACTION CIRCUIT AND COLLECTOR FOR DECELERATING ELEGTRONS Filed July 11, 1963 MiGfJI Buerazv J. 0051 50A/ mwewme,

United States Patent M HIGH EFFICIENCY TRANSVERSE-FIELD TRAVEL- ING WAVE TUBE HAVING FAST WAVE DISSIPA- TIVE COUPLER BETWEEN INTERACTEUN CIR- CUlT AND COLLECTOR FOR DECELERATING ELECTRONS Burton J. Udelson, Bethesda, Md, assignor to the United States of America as represented by the Secretary of the Army Filed July 11, 1963, Ser. No. 294,455 1 Claim. (Cl. 315--3.6)

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to me of any royalty thereon.

This invention relates to traveling wave tubes, and more particularly to a high efficiency transverse-field traveling Wave tube having a depressed potential collector.

In a traveling Wave tube an electromagnetic wave propagates along a wave interaction circuit and an electron stream also passes along the wave interaction circuit in coupling relation with the electric field of the wave. By proper adjustment of the velocity of the electron stream and the phase velocity of the wave, the stream and the wave may be made to interact cumulatively. The transverse-field type amplifier to which this invention relates, and which is exemplified by the patent to P. R. Pierce, Serial No. 2,843,792, differs from the conventional O-type traveling-wave amplifier. In the conventional O-type traveling-wave tube, the electron beam interacts primarily with longitudinal fields. To obtain effective energy transfer from the beam to the RF wave, the electron beam must travel slightly faster than the RF wave on the slow wave structure. Thus, each electron will experience RF fields which alternately act to accelerate it and to decelerate it. To maintain continuity of DC. current flow, the electron density will be greater in the regions where the electron velocity is low, thus forming electron bunches. The bunches remain in synchronism with the RF circuit wave, even though the individual electrons, because of their greater velocity, are moving in and out of these bunches. The net loss of DC. energy by the electron beam stems from the fact that, as the beam exits from the slow wave structure, the number of electrons in the low velocity bunched region exceeds the number in the high velocity anti-bunch region (i.e., more electrons have been slowed down than have been accelerated).

In the transverse-field structure on the other hand, rather than being longitudinally bunched, the electron beam is displaced laterally by the transverse fields into regions in which it is slowed down by the longitudinal fields. The electron beam wave and the RF circuit wave travel at the same velocity, while individual electrons travel along the electron beam path at a slightly greater velocity. Once the proper phase relationship between the electron beam wave and the RF circuit Wave has been established, all the electrons will experience primarily decelerating RF forces, and the beam emerging from the transverse-field slow-wave structure will have a much more uniform longitudinal velocity than the beam emerging from longitudinal field devices.

In general, there are two methods, with a given circuit and beam current, to enhance the efliciency of traveling wave type tubes. The first is to prolong the time of synchronization between the circuit wave and the beam wave. This may be done either by decreasing the RF wave velocity towards the end of the slow wave structure by modifying its periodicity, or by increasing the DC. voltage along the length of the RF structure in some manner so as to maintain a constant beam velocity.

3,302,053 Patented Jan. 31, 1967 A second method for improving the efiiciency of traveling-wave tubes is to try to recover the unspent beam energy by collecting the beam with a collector operating at a low DC. potential relative to that on the slow wave structure. This is the approach of the instant application. The main factor limiting the effectiveness of this method is that the electron beam emerging from a conventional O-type device has a wide range of DC. velocities. That is, difference in velocity between the slowest and fastest electrons, resulting from interaction with the slow wave structure, makes it quite difficult to collect all of the electrons in the beam when the collector voltage is reduced. In theory, a monoenergetic beam can be collected by a depressed collector placed at a DC. potential such that all the beam electrons reach the collector with zero velocity. Where a range of electron velocities exist, however, secondary emissions will occur at the collector.

It is an object of this invention to increase the etficiency of transverse-field microwave tube devices.

Another object of this invention is to increase the efliciency of transverse-field microwave tube while ensuring maximum collection at the collector and minimum reflection of electrons back to the interaction circuit.

A further object of this invention is to increase the cili ciency of transverse-field microwave tubes independently of the gain parameter.

A still further object of the present invention is to pro vide a simple, and inexpensive means to increase the efliciency of transverse-field microwave tubes.

These and other objects of the present inventions are accomplished by providing a fast wave electron beam coupler between the transverse-field amplifier section of the traveling wave tube, and a depressed potential collector. The transverse-field amplifier itself, consists of two parallel planar slow-wave structures. Power is fed to these structures in a push-pull manner so that a transverse RF field is present between the two structures. The power output from the transverse-field amplifier is also push-pull. The fast wave coupler is also of a planar parallel construction. It is terminated by a matched load impedance at both the input and output ends. An electron sheet beam, either magnetically or electrostatically focused, traverses first the transverse-field amplifier region and then the fast wave coupler region. By placing the fast wave coupler after the transverse-field device, the transverse displacement and transverse velocity compo nents present on the beam from the transverse-field structure are substantially removed. Thus, the beam emerging from the fast wave coupler will have had most of its transverse displacement and transverse velocity component removed, in addition to having a highly monoenergetic longitudinal component of velocity, thereby permitting an increase in the efficiency of the tube through the use of beam collection with a depressed collector.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIG. 1 is a partial schematic diagram illustrating the basic concepts of the invention as applied to a magnetically focused transverse-field amplifier.

FIG. 2 is a sectional view of a specific embodiment of the invention as applied to an electrostatically focused transverse-field amplifier.

FIG. 3 is a sectional view along the line 3--3 of FIG. 2.

Referring to FIG. 1, there is a transverse-field travelling wave tube 11 of the magnetically focused type which may be used to illustrate the principles of this invention. The travelling wave tube 11 has as its principle components, an electron gun 12, a transverse-field slow wave structure 13, a fast wave electron beam coupler 14, a magnetic 3 focusing field B, and a depressed potential collector 15. Although the transverse-field amplifier 13 and the fast wave coupler 14 could be fabricated using different types of circuitry, it is usually more convenient from a con struction standpoint to use the same type of structure for both mechanisms.

The amplifier 11 is enclosed in a vacuum tight evacuated glass envelope designated 16. An electron sheet beam 17 from electron gun 12 is injected into the transverse-field slow wave structure 13. The transverse-field slow wave structure 13 includes two opposed parallel planar slow wave structures 18 and 19. The slow wave structures 18 and 19 are fed from the signal source to be amplified in a push-pull manner by means of terminals 21 and 22. The amplified output appears at terminals 23 and 24. The slow wave structures 18 and 19 have the same phase velocities separately, and have phase velocities in combination approximately the same as the velocity of an electron beam 17. The electron beam 17 will have a natural resonant frequency which is dependent upon the type of focusing used, either electrostatic or magnetic.

The Doppler relation that must be obtained in the amplifier section 13, where the RF wave couples to a slow wave on the beam, is:

where,

=the electron beam velocity,

,u=velocity of the RF wave along slow wave structure in the presence of the beam,

w=input frequency (in radians/ second),

w =natura1 resonant beam frequency or cyclotron frequency depending upon whether electrostatic or magnetic focusing is used (in radians/ second).

When the relationship of Equation 1 is maintained, the electron beam 17 is laterally displaced from the center line 26 due to the transverse fields set up by the slow wave structures 18 and 19 which are fed push-pull. The structure and operation of the electron gun 12 and transverse-field amplifier section 13 thus far described are conventional and well known in the art, and the description given here is merely for the sake of completeness and ease of understanding.

In accordance with the teaching of this invention, following the transverse-field amplifier section 13, the electron beam 17 passes through a fast wave coupler 14 before impinging on collector The purpose of the fast wave coupler 14, as previously mentioned, is to remove the transverse displacement and velocity components from the electron beam 17 At the output of the amplifier section 13, the electron beam 17 will have a high monoenergetic longitudinal component of velocity which is inherent to transverse-field travelling wave tubes. The fast wave coupler 14 is of the type used to couple to and from the electron beam in an electron beam type parametric amplifier. Either a resonant type coupler or a distributed parameter type coupler may be used. An example of the resonant type coupler is that described in the article by C. L. Cuccia titled, The Electron Coup1erA Developmental Tube for Modulation and Power Control at Ultra-High Frequency, appearing in the RCA Review, vol. 10, page 278 (June 1949). An example of the distributed parameter type coupler is described in the article by R. H. Pantell titled, Electrostatic Electron Beam Couplers, in the I.R.E. Transactions on Electron Devices, vol. 8, pp. 39-4 3, January 1961.

Both ends of the coupler are terminated with matching impedances 27, the back ends of which dissipate the energy coupled from electron beam 17. The matching impedances, as is well known in the art, may "be formed by imbedding a lossy ceramic material on wires 25 or in any other manner deemed suitable to the particular tube structure used. In order to couple transverse energy on the electron beam 17 to the coupler 14, the following fast wave relationship for the coupler must be satisfied:

it, to

Since 01 and [.L remain the same in the amplifier and coupler sections, Equations 1 and 2 must be satisfied in their respective regions by properly adjusting the value of ,u and w in each region. The electron beam velocity may be varied in each region by adjusting the voltage on the slow wave structure. With magnetic focusing, the natural resonant frequency of the beam may be varied by varying the strength of the magnet focusing field. With electrostatic focusing, the natural resonant beam frequency may be varied by adjusting the electrostatic lens strength.

The power dissipated in the matched load of the coupler 14 may be minimized. As is known in the art, the RF power in the electron beam and in the transversefield circuit in a transverse-field travelling wave tube is:

y=maximum beam displacement from the center plane of the slow wave structure,

I=D.C. beam current,

1;:61601101'1 charge to mass ratio.

Dividing Equation 3 by Equation 4 gives circuit fi beam Thus, if the transverse-field RF beam power fed to the output coupler 14 is to be dissipated in a matched load, this loss of power will be small if w in the transverse-field amplifying section is made much less than to. In transverse-field travelling wave tubes in use, values of to /w between 0.1 and 0.25 are normally used.

The collector may be any desired well known type operated at a depressed potential. Ideally, it should have as low a secondary emission as possible, and in most cases it is desirable to make the collector in the form of an electron trap.

FIGS. 2 and 3, show, in a preferred embodiment, the principles of this invention as applied to an electrostatically focused transverse-field microwave tube. The chief advantage of this embodiment is ease of construction, and the absence of the magnet which is needed in magnetically focused tubes. Here the transverse-field travelling wave tube includes an amplifier section 30 in an evacuated glass envelope 31. The amplifier has two opposed parallel planar slow wave structures 33 and 34, in this embodiment, V-shaped members 36 and 37 (see FIG. 3) having ridge plates 38 and 39 insulated from the V-shaped members 36 and 37 by insulating strips 41 and 42. The broad walls opposite V-shaped members 36 and 37 are ladder lines 43 and 44. The opposed slow wave structures 33 and 34 are held apart a distance sufficient to allow the electron beam to pass between the structures by separators 45.

The ridges 33 and 39 are maintained at a first DC. potential V and the ladder lines 43 and 44 are maintained at a second DC. potential V These periodic voltages V and V furnish the electrostatic focusing lenses.

As is known, electron focusing within the electrostatic system is caused by transverse components of electric field and variations of longitudinal electric field established between the ladder lines 43 and 44. Along the center plane indicated by center line 48, the transverse-field is zero, and it increases with distance from this center line. Consequently, it may be shown that electrons of the beam are subjected to a force which tends to accelerate them away from the center plane 48 in regions of high potential and to accelerate them towards the center plane in low potential regions, so that small ripples are imposed on the electron trajectories. The low potential regions, however, exert the major influence because the electrons are farther from the center plane as they traverse these regions and are consequentially in a stronger field, while at the same time the electrons move more slowly through the low potential region and hence require more time to traverse it. The two effects are equally strong in result with a net force tending to deflect the electrons towards the center plane of the reference path. This efiect is comparable to an elastic force, consequentially, the beam electrons follow approximately simple harmonic motion trajectories centered about the center line 48, with of course, the small ripple caused by the localized electric fields superposed.

An electron beam 49 is formed in an electron gun section which includes a heated cathode 51, beam forming electrode 52, an anode 53, and a collimator 54. The electron beam 49 which emerges from the electron gun is substantially a two dimensional sheet or ribbon beam.

The signal to be amplified is applied to terminals 61 and 62, which then excite what might be described as two U-shaped waveguides defined by ladder 43, ridge 38, and the short walls of U-shaped member 36; and ladder 44, ridge 39,1and the short walls of U-shaped member 37, respectively. The amplified output is then coupled from ladder lines 43 and 44 to terminals 53 and 54, respectively.

Following the amplifier section is a fast wave electron beam coupler section 56 similar to the arrangement shown and described in connection with FIG. 1 with the exception that electrostatic focusing is used, and substantially identical in structure to the amplifier section 30, with like parts having like reference numbers. The fast wave coupler 56 is of the travelling wave type. To satisfy the coupler relation of Equation 2 the potentials V and V are placed on the ladder rungs 46' and ridge plates 38' and 39 in the coupler section. The transverse energy coupled beam in the fast w-ave section is dissipated in lossy ceramics 57 imbedded in the ridge plates 38 and 39. The lossy ceramic inserts 57 are matched to the impedance of the fast Wave coupler section to prevent reflections, as is common in the art.

The electron beam 49, with its transverse energy components removed in the fast wave coupler 56, is then collected in a depressed potential collector 59. As was described in FIG. 1, collector 59 will normally be of a trap type and be covered with a non-emissive coating to reduce secondary emission due to electrons impinging on the collector. The collector 59 is maintained at as low a DC. potential as possible, consistent with collecting all the current.

The principle of eliminating the transverse component of beam velocity and displacement by placing a fast wave coupler behind a transverse-field device may also be applied to the transverse-field backward-wave oscillator as well as to the transverse-field forward amplifiers just discussed. The electrostatically focused transverse-field backward-wave oscillator appears to be particularly well suited to employ the teachings of this invention. In such a device a continuous slow-wave structure may be used both for the backward-wave oscillator and coupler regions, Whereas in the forward-wave tube it is necessary to employ physically separated slow wave structures in order to facilitate coupling of power from the output of the transverse-field amplifier section. The equation that must be satisfied in the backward-wave oscillator region in order to have the electron beam wave velocity (,u.) be in the opposite direction to that of the electron velocity is:

u w where w must be greater than (.0, and in general will be of the order of 2m. This places a restriction on the upper limit of the frequency of oscillation of the transverse-field backward-wave oscillator, because it is difficult physically to make w Very high in either an electrostatically focused or magnetically focused device. The coupler relationship remains the same as for the transverse-field amplifier.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claim.

I claim as my invention:

In a transverse-field microwave tube having an electron gun and a depressed collector electrode spaced apart for defining therebetween a path of electron flow and having an interaction circuit including a pair of slow wave structures where the transfer of DC. electron beam enengy on said slow Wave structure is achieved by having the electron beam travel adjacent to the slow wave structure at a velocity slightly faster than the velocity of the RF wave of the structure, the improvement comprising:

a dissipative fast wave coupler interposed between said interaction region and said depressed collector electrode, where-by transverse velocity and displacement components are removed from said electron beam allowing said beam to be collected at a depressed potential.

No references cited.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

R. D. COHN, Assistant Examiner. 

